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Lennox heat exchanger bad after less than 1yr

Lennox MN LGC036S2BS1G SN 5606B06672 nat gas rooftop, replaced the heat exchanger less than 1 yr ago when it was found bad on a PM. Came back a week ago to do the PM again for this place, checked the heat exchanger just to make sure and it's burnt out in the same spots as before.... thoughts? The first pic is the first time I found it bad and the 2nd pic is from when I went a week ago.

"You better watch who you're calling a child Lois. Because if I'm a child then you know what that makes you? A pedophile, and I'll be damned if I'm gonna stand here and be lectured by a pervert."

Anything stopping or restricting airflow on the flue pipe? I've found huge hornet nests in those things a couple times. Luckily they were all dead when I began to do the heat start up. Have you been doing the maintenance on this unit since 2006? If not, maybe the inducer motor has been replaced prior to the heat exchanger and they messed up with the new insulation for it? Just a thought.

Had the exact thing last year on a 4 ton Lennox RTU. Unit was a 1.5 years old, operated during one winter. That winter was colder than usual for a longer time, so the RTU was worked hard and FINALLY tripped a roll-out.
This is what I figured out.
1) Previous RTU was a 2 ton Rheem, contractor misread the nomenclature and installed the 4ton Lennox with new transition curb.
2) Original contractor did not de-rate the gas as per manufacture instructions for the altitude of 6000 feet.

Correction...
1) Original contractor was out of business
2) Lennox was more than kind enough to warranty HX and a new HX was installed.
3) Added two new returns and 4 more supplies to get 4 tons of air moving, luckily the main plenums were large enough.
4) Properly de-rated gas supply.
5) Back to the basics of commissioning the system by taking statics and making sure the unit was operating within the temperature rise, had to actually drop a motor speed after the changes.

So VERY good chance you have undersized ductwork causing the poor RTU to just cycle on primary limit with a probability of over-firing.

What is your static across the unit?
What is the temperature rise across the unit?

We are going to wait till it gets colder here before we take more tests on the unit, (it was 75 today). The gas pressure was fine 3.5" as it should be, no restrictions in the tubes and the inducer is also fine. We do think it could be a duct issue as they have what seem to be too small returns and the grills were also clogged. I also didn't have the air balancing hood with me to check CFMs or anything. I'll let you know if we find anything, but we figured an airflow or gas issue.

"You better watch who you're calling a child Lois. Because if I'm a child then you know what that makes you? A pedophile, and I'll be damned if I'm gonna stand here and be lectured by a pervert."